It has become common in electronic devices having a display screen such as mobile communication terminals (cellular phones) to configure the display unit with a touch panel wherein the user touches the screen for input. The touch panel allows for a more intuitive input operation and provides a useful input method, particularly to mobile communication terminals having smaller keys.
Many such electronic devices have a liquid crystal panel as the display unit. When a capacitive touch panel is configured in the liquid crystal panel, driving of the liquid crystal panel can be a noise source and may affect capacitance change due to finger touch. As a result, user finger touch may not be properly detected.
The driving of a liquid crystal panel that may be responsible for detection errors varies depending on the display state such as screen design. In other words, the quantity of display noise varies depending on the display screen at the time. For example, in a so-called normally black liquid crystal that is black when no voltage is applied to the R, G, and B colors, there is little fluctuation in the drive voltage when a black screen is displayed because the voltage applied to the pixels is “0.” Conversely, when voltage is applied to G pixels among R, G, and B pixels to display a green screen, the difference in the voltage between adjacent pixels is most remarkable and noise is increased compared with the black screen.
As described above, the quantity of display noise varies depending on the display color constituting the display screen. The tendency for touch panel detection errors to occur on the touch panel affected by display noise is related to the screen design. Therefore, such a relationship can presumably be used to reduce detection errors.
Touch panel detection errors often occur due to influence of the above external factors. Techniques for reducing detection errors, for example, by taking into consideration environmental change such as temperature change (for example, Patent Literature 1) or by changing detection conditions depending on whether the back light is ON or OFF (for example, Patent Literature 2) have been proposed.